如何在Flutter中发现Widget何时出现/消失？

class MyWidgetObserver extends WidgetsBindingObserver {
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    super.didChangeAppLifecycleState(state);
    if (state == AppLifecycleState.resumed) {
      // Widget出现
      print('Widget appeared');
    } else if (state == AppLifecycleState.paused) {
      // Widget消失
      print('Widget disappeared');
    }
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(MyWidgetObserver());
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(MyWidgetObserver());
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      // Widget内容
    );
  }
}